8.2. Redundancy in practice

When configured to operate in redundant mode, PowerMail only considers a message delivered when it is stored on more than 2 servers residing in different groups, and that number can be raised. If less than 2 servers are available, powersmtp starts to refuse mail delivery, giving out temporary 'please try later' errors. This puts the burden of queueing on the delivering agent, where it belongs.

When retrieving mail, all backends are queried for which messages they have listed for that mailbox. In case of drastic failure, messages might start to disappear from a mailbox and return only when backends a restored.

8.2.1. Queued deletions

A backend that has been out of service might on return cause messages that the user has deleted to inadvertently reappear. To prevent this, powerpop stores a failure log per backend which stores failed deletions.

When a backend comes online again, these deletions are first replayed before messages are made available to users again. This prevents messages from magically appearing again.